In clinical chemistry, analyses are conducted on very small samples, so that the accuracy of the results depends closely on the accuracy of the amounts of sample and reagent portions delivered to the area where each is being conducted. This accuracy of amounts depends on the pipettes used to deliver the portions and is correspondingly difficult to guarantee when very small portions of materials are utilized. This has resulted in the development of high-accuracy pipettes but these high-accuracy pipettes are very expensive. In addtion, the pipetting operation has to be repeated for each analyzed sample and each portion of reagent, with the result that the process of preparing a set of samples and reagents for analysis in a rotor of a centrifugal analyzer usually takes longer than the analysis itself.
Another essential factor in the accuracy of analysis is the determination of the time at which the reaction begins, i.e. when the sample is placed in the presence of the reagent. This is the reason why the cells are disposed at the periphery of the analytical rotor, which has chambers for introducing reagent and/or a sample and communicating with the respective cells, the contents of the chambers being simultaneously distributed to the cells by the centrifugal force applied to the liquids upon centrifugation of the rotor.
In U.S. Pat. No. 3,873,217 it has already been proposed to measure out and distribute the portions of the required liquid simultaneously to analytical cells disposed at the rotor periphery. The measuring-out is performed as follows: A liquid injected into a central chamber is divided among various analytical units by equidistant edges or ridges at the chamber periphery. As a result of the rotation of the rotor and the division of the liquid by the equidistant edges, the liquid is forcibly introduced into the units. The aforementioned proportioning lacks accuracy and is dependent inter alia on the amount of liquid introduced into the central chamber, so that the exact quantity has to be pipetted.
According to U.S. Pat. No. 3,744,975, portioning similar to that described in the preceding patent specification is carried out by using an overflow reservoir. The portions are simultaneously distributed to the cells by sending a stream of air under pressure. This does not ensure that a very accurate volume of liquid is transferred.
The aim of the invention is to obviate the disadvantages of the aforementioned methods by providing a high-accuracy portioning of samples and/or reagents followed by simultaneous distribution of the portions, using a device having a simple structure and adapted to be mass-produced at a reasonable price.
The analytical rotor according to the invention is characterized in that liquid-retaining means are associated with the transfer passages to prevent liquid flowing into the analytical cells until the centrifugal force applied to the liquid exceeds a threshold value which is made higher than the hydraulic resistance to flow through the communication passage.
The invention also relates to use of the analytical rotor, characterized in that a volume of liquid greater than the total volume of the portioning cavities is introduced into the distribution chamber, the rotor is driven at a first speed to exert on the liquid a centrifugal force greater than the aforementioned hydraulic resistance to flow through the communication passage but smaller than the threshold value, in order to fill the portioning cavities and discharge the surplus liquid into the overflow reservoir, and the rotor speed is changed to a second speed greater than the first, in order to exert on the liquid a centrifugal force greater than the threshold value, so as to transfer the portions contained in the portioning cavities to the respective analytical cells.